Constructor not considered for lexy::dsl::nullopt

[xsebek]

The documentation of lexy::dsl::nullopt says:

Values

A single object of the tag type lexy::nullopt. This type is implicitly convertible to any type that is optional-like by constructing a default constructible instance of it.

So when I want to create a list using lexy::dsl::opt, I would expect a constructor that takes vector to match, because vector is default constructible. But it does not. Neither does the default constructor or one taking std::nullopt_t.

The only ones that I found to match were std::optional<std::vector<...>> and lexy::dsl::nullopt:
https://godbolt.org/z/c3M7banhf

[foonathan]

Emphasis mine:

This type is implicitly convertible to any type that is optional-like by constructing a default constructible instance of it.

It is only types that are optional-like, not all default constructible types. This restriction is unfortunate, but necessary because otherwise std::optional<int>(lexy::nullopt) would be an optional containing a default-constructed int, not an empty optional.

The particular situation of dsl::opt(dsl::list(...)) is annoying, yeah. It might be worth extending to lexy::nullopt to do implicit conversions to anything that is optional-like (it requires T(), *T() and !T()) or container-like (maybe T() and T().empty()?). However, that is both a breaking change and would mean that std::optional<std::vector<T>>(lexy::nullopt) would be an optional containing an empty container, not an empty optional - which might be confusing? That being said, it matches the current behavior of std::optional<std::optional<T>>(lexy::nullopt).

[xsebek]

@foonathan the thing I would like to avoid is unreadable and unnecessary constructors like this one:

Group(std::optional<std::vector<int>> xs): _xs(xs ? std::move(*xs) : std::vector<int>()) {};

My understanding is that lexy needs either this one constructor or two constructors - one for the empty case and one for the list case. However, the empty case seems only satisfied with lexy::nullopt.

To me the wording of values produced by opt and nullopt seems unclear - the only thing I am sure of is that it supports std::optional.

---

As a side note, I just discovered opt_list in brackets/terminator, which should do what I want. Maybe linking to those as an alternative to opt(list(...)) would help? Or adding an example with a value callback. 🙂

[foonathan]

My understanding is that lexy needs either this one constructor or two constructors - one for the empty case and one for the list case. However, the empty case seems only satisfied with lexy::nullopt.

Yes, you either have single function accepting an optional<vector<T>. or two overloads, one taking lexy::nullopt and one taking vector<T>. I might change that.

To me the wording of values produced by opt and nullopt seems unclear - the only thing I am sure of is that it supports std::optional.

Yes, I will make the docs clearer.

As a side note, I just discovered opt_list in brackets/terminator, which should do what I want. Maybe linking to those as an alternative to opt(list(...)) would help? Or adding an example with a value callback. slightly_smiling_face

opt_list is essentially the same as opt(list(...)); it won't help you.